Microwave switches are used in a variety of applications. For example, in satellite technology, microwave rotary switches (R-switches) and C-switches are widely used as redundant switches to connect a spare device when an active device malfunctions. Typically, large numbers of R-switches and C-switches are employed in a satellite system.
FIG. 1 illustrates the cross-section of a typical microwave R-switch assembly 10 includes a housing 2 (also known as a “stator”) having waveguide ports 14A, 14B, 14C and 14D and a hollow cylindrical interior 16, and a cylindrical rotor 18 within the housing 2. Rotor 18 typically has three waveguide paths, a straight central waveguide passage 11, and two curved waveguide passages 8 and 12 that connect various waveguide ports depending on the specific position of rotor 18 within housing 2. An actuator (not shown) is used to move the rotor to various predetermined positions. Also, in microwave R-switches and C-switches, it is necessary to provide a physical clearance gap between the rotor and the housing so that the rotor may be rotated within the housing. As shown, a physical clearance gap G between the outer surface of the rotor 18 and the inner surface of the housing 2 exists to allow the rotor 18 to rotate unobstructed within housing 2.
When an electromagnetic signal is propagating from a connected port 14B at one end of a switched-through waveguide passage 11 to another connected port 14D at the other end of the waveguide passage 11, leakage of some of the electromagnetic signal through clearance gap G typically causes the unconnected ports 14A and 14C to show an electromagnetic signal, thus degrading the isolation and the insertion loss performance of the microwave switch. Essentially, the gap G acts as a transmission line and since the gap G encompasses the entire circumference of the rotor 18, the electromagnetic signal can be indicated at various ports within housing 2. Also, the not-switched-through waveguide passages 8 and 12 and the inner surface of housing 2 adjacent to waveguide passages 8 and 12 form a volume resonator. If the frequency of the signal passing through the switched-through waveguide passage 11 is close to the resonant frequency of these volume resonators, a signal will appear at the unconnected ports 14A and 14C characterized by a spurious narrow spike in the isolation and insertion loss characteristics around the resonant frequency.
It is important to achieve a high degree of mutual isolation of unconnected ports 14A, 14B, 14C and 14D. For example, in the case of redundancy circuit networks for application within satellite systems, the ratio of the power occurring at a port that is not connected any other port (e.g. 14A), to the power supplied to a port (e.g. 14B) that is connected with another port (e.g. 14D), should be at least as low as approximately −60 dB. This power ratio requirement is applicable to R-switches having any number of ports 14. Mutual isolation of unconnected ports is conventionally achieved in two ways.
One approach is to narrow the gap G in order to reduce the electromagnetic signal leakage through gap G. However, this approach is limited by mechanical and thermal requirements and reliability concerns. Specifically, if the gap is narrowed too much, it is not possible to provide housing assembly 10 that functions at an acceptable level over a reasonable range of operating temperatures due to thermal expansion characteristics of rotor 18 and housing 2.
Another approach is to provide longitudinal and circumferential grooves on the surface of the rotor and/or by providing grooves on the inner surface of the housing. For example, in U.S. Pat. No. 3,155,923 to Persson, U.S. Pat. No. 4,649,355 to Ullman, and U.S. Pat. No. 6,218,912 to Mayer, the isolation of unconnected ports can be improved using such methods and result in a ratio of even less than −60 dB. However, the use of such grooves on the inner surface of the housing does not appear in practice to eliminate the appearance of the spurious narrow spike. The inventors have determined that in practice, the spurious narrow spike still can have an amplitude in the range −35 to −40 dB. In addition, the provision of longitudinal and circumferential grooves adds to the complexity and manufacturing cost of producing housing assembly 10.